#include<stdio.h>
int main()
{
int x, a, y, b;
float c = 0;

	scanf_s(" %d %d %d %d", &x, &a, &y, &b);
	c = (b * y - x * a) / (b - a);
	
		
		printf("%.2f\n", c);
		return 0;


}